Internet Protocol (IP) phone services have come into wide use for supporting a call between terminals which are installed with Voice over Internet Protocol (VoIP). In IP phone services, packetization of audio or voice signals and procedures relating to routing are executed in accordance with protocols, such as User Datagram Protocol (UDP) or IP, belonging in the transport Layer or the internet layer of the Transmission Control Protocol/Internet Protocol (TCP/IP). A user may access such services through a network device, such as a router, a repeater hub or a switching hub by only installing an application compliant with Session Initiation Protocol or H. 323, where these protocols manage call transmissions or call receptions at the high layer, in a user's terminal connected to the network device (Refer to the patent document 1 or 2 for detailed descriptions of SIP and H. 323).    [Patent Document 1] JP 2005-160055 A    [Patent Document 2] JP 2005-191881 A